Recombinant protein production is an essential activity for high throughput screening, functional validation, structural biology, and production of pharmaceutical polypeptides. Escherichia coli is a widely used organism for the expression of heterologous proteins because it easily grows to a high cell density on inexpensive substrates, and has well-established genetic techniques and expression vectors. However, this is not always sufficient for the efficient production of active biomolecules. In order to be biologically active, polypeptide chains have to fold into the correct native three-dimensional structure, including the appropriate formation of disulfide bonds, and may further require correct association of multiple chains.
Although the active state of the protein may be thermodynamically favored, the time-scale for folding can vary from milliseconds to days. Kinetic barriers are introduced, for example, by the need for alignment of subunits and sub-domains. And particularly with eukaryotic proteins, covalent reactions must take place for the correctly folded protein to form. The latter types of reaction include disulfide bond formation, cis/trans isomerization of the polypeptide chain around proline peptide bonds, preprotein processing and the ligation of prosthetic groups. These kinetic limitations can result in the accumulation of partially folded intermediates that contain exposed hydrophobic ‘sticky’ surfaces that promote self-association and formation of aggregates.
Antibodies are tetrameric proteins, which have many uses in clinical diagnosis and therapy. Each antibody tetramer is composed of two identical light chains and two identical heavy chains. Pure human or humanized antibodies of a specific type are difficult or impossible to purify in sufficient amounts for many purposes from natural sources. As a consequence, biotechnology and pharmaceutical companies have turned to recombinant DNA-based methods to prepare them on a large scale. The production of functional antibodies requires not just the synthesis of the two polypeptides but also a number of post-translational modifications, including proteolytic processing of the N-terminal secretion signal sequence; proper folding and assembly of the polypeptides into tetramers; formation of disulfide bonds; and specific N-linked glycosylation. All of these events take place in the eukaryotic cell secretory pathway, an organelle complex unique to eukaryotic cells.
Recombinant synthesis of such complex proteins has had to rely on higher eukaryotic tissue culture-based systems for biologically active material. However, mammalian tissue culture based production systems are significantly more expensive and complicated than microbial fermentation methods. In addition, there continues to be questions regarding therapeutic products produced using materials derived from animal by-products.
As a eukaryote, Pichia pastoris has many of the advantages of higher eukaryotic expression systems such as protein processing, protein folding, and posttranslational modification, while being as easy to manipulate as E. coli or Saccharomyces cerevisiae. It is faster, easier, and less expensive to use than other eukaryotic expression systems such as baculovirus or mammalian tissue culture, and generally gives higher expression levels. As a yeast, it shares the advantages of molecular and genetic manipulations with Saccharomyces. These features make Pichia very useful as a protein expression system.
Many of the techniques developed for Saccharomyces may be applied to Pichia. These include transformation by complementation; gene disruption and gene replacement. In addition, the genetic nomenclature used for Saccharomyces has been applied to Pichia. There is also cross-complementation between gene products in both Saccharomyces and Pichia. Several wild-type genes from Saccharomyces complement comparable mutant genes in Pichia. 
Heterologous expression in Pichia pastoris can be either intracellular or secreted. Secretion requires the presence of a signal sequence on the expressed protein to target it to the secretory pathway. While several different secretion signal sequences have been used successfully, including the native secretion signal present on some heterologous proteins, success has been variable. A potential advantage to secretion of heterologous proteins is that Pichia pastoris secretes very low levels of native proteins. That, combined with the very low amount of protein in the minimal Pichia growth medium, means that the secreted heterologous protein comprises the vast majority of the total protein in the medium and serves as the first step in purification of the protein.
Many species of yeast, including Pichia, are mating competent. This enables two distinct haploid strains to mate naturally and generate a diploid species possessing two chromosomal copies.
Although P. pastoris has been used successfully for the production of various heterologous proteins, e.g., hepatitis B surface antigen (Gregg et al. (1987) Bio/Technology 5:479), lysozyme and invertase (Digan et al., (1988) Dev. Indust. Micro. 29:59; Tschopp et al., (1987) Bio/Technology 5:1305), endeavors to produce other heterologous gene products in Pichia, especially by secretion, have given mixed results. At the present level of understanding of the P. pastoris expression system, it is unpredictable whether a given gene can be expressed to an appreciable level in this yeast or whether Pichia will tolerate the presence of the recombinant gene product in its cells. Further, it is especially difficult to foresee if a particular protein will be secreted by P. pastoris, and if it is, at what efficiency.
The present invention provides improved methods for the secretion of heterologous heteromultimers from mating competent yeast, including Pichia species.